Optical devices used in PON (passive optical network) systems which realize economical optical access systems are required to be increased in bandwidth because PON systems handle optical signals in the 1.3-μm band or the 1.55-μm band. Widening of a bandwidth used is also required of DWDM (dense wavelength division multiplex) communication systems which multiplex and transmit optical signals and are used in high-speed, large-capacity core optical communication networks.
Optical switches are known that are equipped with plural optical fibers disposed in a capillary, a planar mirror for deflecting light emitted from an optical fiber, and a lens for optically coupling the set of optical fibers and the planar mirror to each other (refer to JP-A-2004-070050, for example).
However, in the above optical switches, the optical characteristic of the optical switch may vary due to wavelength dependence of the focal length of the collimating lens. The wavelength dependence of the focal length of the collimating lens results from the fact that the refractive index of the collimating lens depends on the wavelength of received light.
On the other hand, optical fiber coupling systems are known in which the wavelength dependence of the focal length of a collimating lens is suppressed (refer to JP-A-2004-020720, for example). In particular, it is disclosed that the wavelength dispersion of the collimating lens in an infrared range is suppressed by paying attention to an infrared dispersion index x and a refractive index of a glass material of the collimating lens at a wavelength 1.55 μm. Like the Abbe number, the infrared dispersion index x represents wavelength dispersion of a glass material in an infrared range.
However, in, for example, the optical fiber coupling system disclosed in JP-A-2004-020720, the focal length of the collimating lens is relatively short (f=1.5 mm) and the collimating lens, a semiconductor laser, and an optical fiber are optically coupled with each other in such a manner that their optical axes approximately coincide with each other.
Therefore, no consideration is now being given to application of this optical fiber coupling system to an optical fiber coupling system that uses a lens having a relatively great focal length or an optical fiber coupling system in which light shines on and exits from an off-axis position of a collimating lens. In these cases, the wavelength dispersion of the optical characteristic of the optical fiber coupling system becomes noticeable.